Characterization of Graphene Films Grown on Cu-Ni Foil by XPS and LEED
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Previous studies have shown that monolayer graphene films can be grown on Cu substrates by the catalytic decomposition of methane molecules. The solubility of carbon in Cu is negligible at the growth temperatures typically used for graphene growth. This results in the formation of films that self- terminate at a monolayer coverage since there is very little carbon in the sub-surface region that can precipitate to the graphene overlayer during the cooling phase. In an attempt to enhance the catalytic activity of the surface and to grow multilayer graphene rather than solely monolayer graphene, use of a 70 -30 Cu-Ni alloy foil has been investigated. The growth of the graphene was performed in a home-made, cold- wall, chemical vapor deposition system by the research group of Prof. Rodney S. Ruoff at the University of Texas at Austin. As part of this research project, analysis of the surface alloy composition during the different phases of growth was done using X-ray photoelectron spectroscopy measurements, and analysis of the surface structure was done using low energy electron diffraction. Measurements were performed on the Cu-Ni foil before anneal, after anneal in H2, and after growth of graphene in a CH4 environment. XPS measurements were made at both normal emission, and at an exit angle of 50 to enhance the surface sensitivity. Before anneal, the measurements indicate that the surface is Ni-rich and heavily oxidized. After annealing in H2, only a small amount of oxide remained, and the alloy fraction of the surface region was 79/21 Cu- Ni. This indicates that the outermost layer of atoms is most likely Cu. Growth of the graphene overlayer resulted in an increase of the Ni composition of the surface region to 28%, with only trace amounts of oxygen present. Low energy electron diffraction measurements of the foils showed only diffuse background for the Cu-Ni foils before anneal and after anneal in H2. The diffraction measurements of the foil after graphene growth showed diffraction spots and ring structures at 70 eV, which are attributed to the formation of multidomain graphene.